Earth Had Two Moons That Crashed to Form One, Study Suggests

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A tiny second moon may once have orbited Earth before
catastrophically slamming into the other one, a titanic clash
that could explain why the two sides of the surviving lunar
satellite are so different from each other, a new study suggests.

The second moon around Earth would have been about 750 miles
(1,200 kilometers) wide and could have formed from the same
collision between the planet and a Mars-sized object that
scientists suspect helped create the moon we see in the sky
today, astronomers said.

The gravitational tug of war between the Earth and moon slowed
the rate at which it whirls, such that it now always shows just
one side to Earth. The far
side of the moon remained a mystery for centuries until 1959,
when the Soviet Luna 3 spacecraft first snapped photos of it.
(The far side is sometimes erroneously called the dark side, even
though it has days and nights just like the near side.) [ Video: How the Moon
Was Made ]

The moon has two faces

The moon's far side is very different than its near side.

For instance, widespread plains of volcanic rock called "maria"
(Latin for seas) cover much of the near hemisphere, but only a
few maria are seen on the far one. In addition, while the surface
of the near side is mostly low and flat, the
far side is often high and mountainous, with the lunar
surface elevated 1.2 miles (1.9 km) higher on average on the far
side.

Now computer simulations hint a second moon essentially pancaked
itself against its larger companion, broadly explaining the
differences seen between the near and far sides. [ 10
Coolest Moon Discoveries ]

Their calculations suggest this second moon would have formed at
the same time as our moon. Scientists have suggested that our
moon was born from massive amounts of debris left over from a
giant impact Earth suffered from a Mars-size body early on in the
history of the solar system. Spare rubble might also have
coalesced into another companion moon, one just 4 percent its
mass and about 750 miles wide, or one-third of our moon's
diameter.

Earth's second moon

To imagine where this other moon once was, picture the Earth and
the moon as being two points in a triangle whose sides are equal
in length.

The other point of such a triangle is known as a Trojan point, or
a Lagrangian point, named after the mathematician who discovered
them. At such a point, the gravitational attraction of the Earth
and moon essentially balances out, meaning objects there can stay
relatively stably. The
Earth and moon have two Trojan points, one leading ahead of
the moon, known as the L-4 point of the system, and one trailing
behind, its L-5 point.

The researchers computed that this second moon could have stayed
at a Trojan point for tens of millions of years. Eventually,
however, this Trojan moon's orbit would have destabilized once
our moon's orbit expanded far enough away from Earth.

The resulting collision would have been relatively slow at 4,500
to 6,700 miles per hour (7,200 to 10,800 kph), leading its matter
to splatter itself across our moon as a thick extra layer of
solid crust tens of miles thick instead of forming a crater.

"It is entirely plausible for a Trojan moon to have formed in the
giant impact, and for it to go unstable after 10 million to 100
million years and leave its imprint on the moon," study coauthor
Erik Asphaug, a planetary scientist at the University of
California, Santa Cruz, told SPACE.com. Imagine "a ball of
Gruyere colliding into a ball of cheddar."

Moon crash post mortem

The remains of this Trojan moon would make up the highlands now
seen on our moon's far side. At the same time, the impact would
have squished an underground ocean of magma toward the near side,
explaining why phosphorus, rare-earth metals and radioactive
potassium, uranium and thorium are concentrated in the crust
there.

A number of explanations have been proposed for the far side's
highlands, including one suggesting that gravitational
forces were the culprits rather than an impact from Francis
Nimmo at the University of California, Santa Cruz, and his
colleagues. Nimmo said that for now there is not enough data to
say which of the proposals offers the best explanation for this
lunar contrast. "As further spacecraft data and, hopefully, lunar
samples are obtained, which of these two hypotheses is more
nearly correct will become clear," Nimmo said in a statement.

Asphaug and his colleague Martin Jutzi at the University of Bern
in Switzerland detailed their findings in the August 4 issue of
the journal Nature.

Follow SPACE.com contributor Charles Q. Choi on Twitter @cqchoi. Visit
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